Two-dimensional Wind Tunnel Research Articles

Laboratory study on effect of vegetation in reducing wave overtopping under wind effect

Understanding and mitigating wave overtopping are crucial for hinterland safety, yet research primarily focuses on designing coastal structures, with limited attention to structural materials. As global warming intensifies coastal hazards, there is a growing need for resilient infrastructures. A hybrid infrastructure incorporating vegetation alongside concrete may offer a viable solution. In this study, we evaluated the effect of vegetation on wave overtopping reduction under strong winds through hydraulic model experiments simulating extreme storm conditions. The experiments were conducted under the simultaneous action of wind, waves, and vegetation using a two-dimensional wind tunnel flume and vegetation. A higher vegetation density was found to exponentially reduce the overtopping rate. Moreover, dense vegetation was demonstrated to be equivalent to raising the height of protective structures by at least 1.1 m. Furthermore, the increase in overtopping rate by the effects of wind can be compensated by the reduction effects of vegetation. Based on these investigations, an equation was proposed to predict the wave overtopping rate that considers the effects of both wind and vegetation. The results highlight the potential applicability of vegetated coastal protection in future coastal disaster prevention against extreme events with stronger winds and waves that may be induced by climate change.

Experimental design for a novel co-flow jet airfoil

The Co-flow Jet (CFJ) technology holds significant promise for enhancing aerodynamic efficiency and furthering decarbonization in the evolving landscape of air transportation. The aim of this study is to empirically validate an optimized CFJ airfoil through low-speed wind tunnel experiments. The CFJ airfoil is structured in a tri-sectional design, consisting of one experimental segment and two stationary segments. A support rod penetrates the airfoil, fulfilling dual roles: it not only maintains the structural integrity of the overall model but also enables the direct measurement of aerodynamic forces on the test section of the CFJ airfoil within a two-dimensional wind tunnel. In parallel, the stationary segments are designed to effectively minimize the interference from the lateral tunnel walls. The experimental results are compared with numerical simulations, specifically focusing on aerodynamic parameters and flow field distribution. The findings reveal that the experimental framework employed is highly effective in characterizing the aerodynamic behavior of the CFJ airfoil, showing strong agreement with the simulation data.

Influence of deep gaps on laminar–turbulent transition in two-dimensional boundary layers at subsonic Mach numbers

Systematic experimental investigations on the influence of deep gaps on the location of laminar–turbulent transition are reported. The tests were conducted in the Cryogenic Ludwieg–Tube Göttingen, a blow-down wind tunnel with good flow quality, at eight different unit Reynolds numbers ranging from Re_1 = {17.5,,times 10^{6},mathrm{{m}^{-1}}} to 80,times,10^{6},mathrm{m}^{-1}, three Mach numbers, M= 0.35, 0.50 and 0.65, and various pressure gradients. A flat-plate configuration, the extended two-dimensional wind tunnel model PaLASTra was modified in order to allow the installation of gaps with nominal widths of 30 upmu m, 100 upmu m and 200 upmu m and a depth of d = {9,mathrm{mm}}. A maximum Reynolds number based on the gap width Re_{w} = Re_1 cdot w approx {16{,}000} was reached. Transition Reynolds numbers ranging from Re_{tr}approx 1 × 106 to 11 × 106 were measured, as a function of gap width, pressure gradient and Mach and Reynolds number. This systematic investigation facilitates a linear approximation of Re_{tr} dependent on the boundary layer shape factor H_{12} for various flow conditions and gap widths. It was therefore possible to conduct an investigation of Re_{tr} depending on Re_{1} and the relative change of the transition location depending on the gap width w. Incompressible linear stability analysis was used to calculate amplification rates of Tollmien–Schlichting waves and determine critical N-factors by correlation with measured transition locations. The change in the critical N-factor varDelta N by installation of the gap is investigated as a function of w and Re_w. It was found that a gap width of 30 upmu m reduces the critical N-factors in the range of varDelta N approx 0.5 pm 0.25, while gap widths of 100 upmu m and 200 upmu m reduce the critical N-factor in the range of varDelta N approx 1.5 pm 1. Interestingly, an increase in gap width from 100 to 200 upmu m was not found to induce smaller transition Reynolds numbers or reduced N-factors, which might be due to resonance effects.

Measurement Correction of Two-Dimensional Wind Tunnel with Porous Walls using Singularity Method

Measurement Correction of Two-Dimensional Wind Tunnel with Porous Walls using Singularity Method

A Critical Analysis on Low-Order Simulation Models for Darrieus Vawts: How Much Do They Pertain to the Real Flow?

To improve the efficiency of Darrieus wind turbines, which still lacks from that of horizontal-axis rotors, computational fluid dynamics (CFD) techniques are now extensively applied, since they only provide a detailed and comprehensive flow representation. Their computational cost makes them, however, still prohibitive for routine application in the industrial context, which still makes large use of low-order simulation models like the blade element momentum (BEM) theory. These models have been shown to provide relatively accurate estimations of the overall turbine performance; conversely, the description of the flow field suffers from the strong approximations introduced in the modeling of the flow physics. In this study, the effectiveness of the simplified BEM approach was critically benchmarked against a comprehensive description of the flow field past the rotating blades coming from the combination of a two-dimensional (2D) unsteady CFD model and experimental wind tunnel tests; for both data sets, the overall performance and the wake characteristics on the midplane of a small-scale H-shaped Darrieus turbine were available. Upon examination of the flow field, the validity of the ubiquitous use of induction factors is discussed, together with the resulting velocity profiles upstream and downstream the rotor. Particular attention is paid on the actual flow conditions (i.e., incidence angle and relative speed) experienced by the airfoils in motion at different azimuthal angles, for which a new procedure for the postprocessing of CFD data is here proposed. Based on this model, the actual lift and drag coefficients produced by the airfoils in motion are analyzed and discussed, with particular focus on dynamic stall. The analysis highlights the main critical issues and flaws of the low-order BEM approach, but also sheds new light on the physical reasons why the overall performance prediction of these models is often acceptable for a first-design analysis.

The influence of wing twist on pressure distribution and flow topology

Empirical data serve as the foundation to computational modeling in the initial stages of the wind turbine design process. In case of aerodynamic simulations, the empirical input is comprised of lift and drag data obtained in quasi two-dimensional wind tunnel tests. In the simulations, the global flow over an entire blade is finally approximated as a spatial summation of the obtained 2-D data, which stands in strong contrast to the true operation of a wind turbine and consequently leads to a higher level of uncertainty. Especially, the near-root region of the blade experiences highly three-dimensional flow conditions, particularly in regions of the blade where the flow separates from the airfoil. This study aims to accentuate the difference between airfoil data obtained in quasi two-dimensional wind tunnel tests compared to airfoil data from a wing with an imposed three-dimensional spanwise pressure gradient. For this, a geometrically altered wing section with a spanwise twist is tested in a wind tunnel and compared to CFD computations.

Unit Reynolds number, Mach number and pressure gradient effects on laminar\u2013turbulent transition in two-dimensional boundary layers

The influence of unit Reynolds number (Re_1=17.5times 10^{6}–80times 10^{6},text {m}^{-1}), Mach number (M= 0.35–0.77) and incompressible shape factor (H_{12} = 2.50–2.66) on laminar–turbulent boundary layer transition was systematically investigated in the Cryogenic Ludwieg-Tube Göttingen (DNW-KRG). For this investigation the existing two-dimensional wind tunnel model, PaLASTra, which offers a quasi-uniform streamwise pressure gradient, was modified to reduce the size of the flow separation region at its trailing edge. The streamwise temperature distribution and the location of laminar–turbulent transition were measured by means of temperature-sensitive paint (TSP) with a higher accuracy than attained in earlier measurements. It was found that for the modified PaLASTra model the transition Reynolds number (Re_{text {tr}}) exhibits a linear dependence on the pressure gradient, characterized by H_{12}. Due to this linear relation it was possible to quantify the so-called ‘unit Reynolds number effect’, which is an increase of Re_{text {tr}} with Re_1. By a systematic variation of M, Re_1 and H_{12} in combination with a spectral analysis of freestream disturbances, a stabilizing effect of compressibility on boundary layer transition, as predicted by linear stability theory, was detected (‘Mach number effect’). Furthermore, two expressions were derived which can be used to calculate the transition Reynolds number as a function of the amplitude of total pressure fluctuations, Re_1 and H_{12}. To determine critical N-factors, the measured transition locations were correlated with amplification rates, calculated by incompressible and compressible linear stability theory. By taking into account the spectral level of total pressure fluctuations at the frequency of the most amplified Tollmien–Schlichting wave at transition location, the scatter in the determined critical N-factors was reduced. Furthermore, the receptivity coefficients dependence on incidence angle of acoustic waves was used to correct the determined critical N-factors. Thereby, a found dependency of the determined critical N-factors on H_{12} decreased, leading to an average critical N-factor of about 9.5 with a standard deviation of sigma approx 0.8.

Computational modeling of the flow in a wind tunnel

PurposeThe purpose of this paper is to use a computational technique to simulate the flow in a two-dimensional (2D) wind tunnel where the effect of the solid walls facing the model has been addressed using a porous geometry so that interference arriving at the solid walls are duly damped and a flow suction procedure has been adopted at the side wall to minimize the span-wise effect of the growing side wall boundary layer.Design/methodology/approachA CFD procedure based on discretization of the Navier–Stokes equations has been used to model the flow in a rectangular volume with appropriate treatment for solid walls of the confined volume in which the model is placed. The rectangular volume was configured by stacking O-Grid sections in a span-wise direction using geometric growth from the wall. A porous wall condition has been adapted to counter the wall interference signatures and a separate suction procedure has been implemented for reducing the side wall boundary layer effects.FindingsIt has been shown that through such corrective measures, the flow in a wind tunnel can be adequately simulated using computational modeling. Computed results were compared against experimental measurements obtained from IAR (Institute for Aerospace, Canada) and NAL (National Aeronautical Laboratory, Japan) to show that indeed appropriate corrective means may be adapted to reduce the interference effects.Research limitations/implicationsThe solutions seemed to converge a lot better using relatively coarser grids which placed the shock locations closer to the experimental values. The finer grids were more stiff to converge and resulted in reversed flow with the two equation k-w model in the region where the intention was to draw out the fluid to thin down the boundary layer. The one equation Spalart–Allmaras model gave better result when porosity and wall suction routines were implemented.Practical implicationsThis method could be used by industry to point check the results against certain demanding flow conditions and then used for more routine parametric studies at other conditions. The method would prove to be efficient and economical during early design stages of a configuration.Originality/valueThe method makes use of an O-grid to represent the confined test section and its dual treatment of wall interference and blockage effects through simultaneous application of porosity and boundary layer suction is believed to be quite original.

Influence of particle size on inertial particle separator efficiency

Previously reported experiments of inertial particle separator (IPS) geometries have focused on standardized sand mixtures that are a particle mixture of various materials, shapes and sizes. These factors make it difficult to understand the effect of particle size and material properties on overall separator efficiency, effects known to be critical to system performance. To address this problem, spherical glass particles with three different nominal diameters were introduced into a two-dimensional wind tunnel configuration representing the critical flow features of an IPS. The facility was used to investigate three different Outer Surface Geometries (OSGs) as well as three different flow splits, i.e. ratios of scavenge mass flow rate to total mass flow rate. The results indicated that large particles with nominal diameters of 120μm yielded near 100% efficiency indicating that these particle trajectories are dominated by inertia and wall reflections. As particles became smaller, their separation efficiency decreased, especially at low flow splits. This is attributed to an increased influence of aerodynamic effects associated with particle drag and the OSG. These results were then used to validate a model for determining separation efficiency as a function of particle diameter and flow split. Using this model, experimental and computational data found in the literature are compared, identifying critical particle and flow characteristics for IPS design.

A Modified Beddoes–Leishman Model for Unsteady Aerodynamic Blade Load Computations on Wind Turbine Blades

A modified version of the Beddoes–Leishman (B-L) dynamic stall model is presented. A novel approach was applied for deriving the effective flow separation points using two-dimensional (2D) static wind tunnel test data in conjunction with Kirchhoff's model. The results were then fitted in a least-squares sense using a new nonlinear model that gives a better fit for the effective flow separation point under a wide range of operating conditions with fewer curve fitting coefficients. Another model, based on random noise generation, was also integrated within the B-L model to simulate the effects of vortex shedding more realistically. The modified B-L model was validated using 2D experimental data for the S809 and NACA 4415 aerofoils under both steady and unsteady (oscillating) conditions. The model was later embedded in a free-wake vortex model to estimate the unsteady aerodynamic loads on the NREL Phase VI rotor blades consisting of S809 aerofoils when operating under yawed rotor conditions. The results in this study confirm the effectiveness of the proposed modifications to the B-L method under both 2D and three-dimensional (3D) (rotating) conditions.

Two-dimensional wind tunnel measurement corrections by the singularity method

Two-dimensional wind tunnel measurement corrections by the singularity method

Performance of wing sail with multi element by two-dimensional wind tunnel investigations

Performance of wing sail with multi element by two-dimensional wind tunnel investigations

Wind tunnel analysis of the aerodynamic loads on rolling stock over railway embankments: the effect of shelter windbreaks.

Wind-flow pattern over embankments involves an overexposure of the rolling stock travelling on them to wind loads. Windbreaks are a common solution for changing the flow characteristic in order to decrease unwanted effects induced by the presence of cross-wind. The shelter effectiveness of a set of windbreaks placed over a railway twin-track embankment is experimentally analysed. A set of two-dimensional wind tunnel tests are undertaken and results corresponding to pressure tap measurements over a section of a typical high-speed train are herein presented. The results indicate that even small-height windbreaks provide sheltering effects to the vehicles. Also, eaves located at the windbreak tips seem to improve their sheltering effect.

수상 태양광발전 시스템의 풍력계수 산정에 관한 실험적 연구

In recent years, green house effect related natural disasters occur throughout the world. Carbon dioxide, mainly comes from the fossil fuel burning, is suspected to be the cause of green house effect. To reduce the emission of carbon dioxide, we need to find alternative energy resources such as photovoltaic energy. In this paper, the basic characteristics of wind force coefficient on a PV panel installed on the floating type PV energy generation system are investigated though the two-dimensional wind tunnel tests. Test variables included the angle of PV panel, direction of wind, number of rows of PV panel and attached or not attached frame. Based on the results obtained through the wind tunnel tests, it was found that the wind force coefficient can be used as a preliminary data in the design of the structure.

The critical velocity for aggregate blow-off from a built-up roof

A new experimental technique is presented for calculating the critical conditions under which loose gravel on a roof becomes airborne. The critical condition for gravel blow-off from the top of a roof depends on the building geometry, particle properties, and the wind conditions. A series of two-dimensional wind tunnel tests were run to measure the critical condition for particle removal. The experimental results demonstrate that the critical condition for blow-off, parameterized in terms of a particle densimetric Froude number, is a function of the dimensionless particle size (d⁎) and the building geometry. Results for buildings without a parapet show that the critical particle densimetric Froude number has a power-law relationship with the dimensionless particle size as Frd2=8.1(d⁎)−0.44 for the range of parameters tested. For buildings with a parapet, the densimetric Froude number for the critical condition depends on both Reynolds number and parapet height to building height ratio. The experimental results indicate that buildings without a parapet are not always the most prone to blow-off, and that under certain conditions, a small parapet height can increase the risk of gravel removal.As the critical Froude number is dependent on the Reynolds number, raw data from small scale experiments cannot simply be scaled up by using a Froude number. Further, it is demonstrated that the current approach of using the Shields Diagram (or equivalent data) to scale results from small to large scale are also flawed as the motion initiation mechanism is different. Therefore, existing design guides should be re-visited and full-scale experiments should be conducted in order to fully analyze the risk of blow-off.

Scaling between Wind Tunnels–Results Accuracy in Two-Dimensional Testing

The establishment of exact two-dimensional flow conditions in wind tunnels is a very difficult problem. This has been evident for wind tunnels of all types and scales. In this paper, the principal factors that influence the accuracy of two-dimensional wind tunnel test results are analyzed. The influences of the Reynolds number, Mach number and wall interference with reference to solid and flow blockage (blockage of wake) as well as the influence of side-wall boundary layer control are analyzed. Interesting results are brought to light regarding the Reynolds number effects of the test model versus the Reynolds number effects of the facility in subsonic and transonic flow.

Pressure Measurements in Jet Diffusion Using a Developed Hot-Wire Static Pressure Sensor

The aim of this experimental study is to investigate the diffusion of jets, with particular attention focused on the relationship between the static pressure and the stream wise mean velocity on the development of a two-dimensional air jet. A jet with three injection Reynolds numbers (Re=1.0×104, 2.0×104, 3.0×104) was generated in a two-dimensional wind tunnel. The velocity distributions were measured by an X-type hot-wire anemometer. The static pressure distributions were measured by a static pressure probe developed in our laboratory, which incorporates a hot-wire anemometer. There are two important characteristics for the hot-wire static pressure probe. The one is that mean static pressure and the static pressure fluctuation can be measured at the same time. The other is that the probe is not affected by the flow and the changes of physical properties (thermal conductivity) of gases, because the sensing hot-wire is always in the air bias flow. The sensitivity of the fstatic pressure probe is 92.3 mV/Pa. The frequency response is flat from 16 Hz to 2.5 kHz. As a result of the experiment, it was found that negative static pressure exists in the turbulent shear layer. It is considered that the entrainment processes from the negative static pressure by the vortex structure motion of the turbulent shear layer.

Hotwire Measurement and Numerical Analysis of Flows around a Straight Wing Vertical Axis Wind Turbine

This paper describes the analysis of flows around a straight wing vertical axis wind turbine. Hotwire measurements in two-dimensional wind tunnel and numerical simulations are conducted. The rotor has two blades, whose airfoil is NACA0012 and the chord length is 0.1 of the rotor diameter. Reynolds number based on the uniform velocity and rotor diameter is 8.0 × 104. The tip speed ratio is 2.0. Incompressible Navier-Stokes equation is used for numerical simulation. A rotating coordinate system, which rotates at the same speed of the rotor, is employed. Both the experimental and numerical results are compared. As a result, the velocity fields show good agreements at each rotation angle, and the aerodynamic torque varies according to the flow condition, especially the flow separation.

Two-Dimensional Low-Reynolds Number Wind Tunnel Results for Airfoil NACA 0018

The two-dimensional characteristics of airfoil NACA 0018 have been measured for Reynolds numbers between 0.15×106 and 1.0×106 to establish the lift, drag and moment curves that serve as input to performance calculations of vertical axis wind turbines. At the lower surface laminar separation occurs at low to medium angles of attack, which is of significant influence on the characteristics and the radiated noise. For the situation with a lower surface laminar separation bubble, span wise wake rake traverse measurements showed an irregular three-dimensional pattern. Noise reduction could be achieved with zigzag tape at the 70% to 80% lower surface chord station. Significant post-stall hysteresis loops occurred showing a high loss in lift.

EFFECT OF OPPOSED FLOW ON FLAME SPREAD OVER A FINITE-LENGTH PMMA SLAB IN A TWO-DIMENSIONAL WIND TUNNEL

The ignition delay and subsequent downward flame spread over a finite-length PMMA slab with the effect of radiation included in a two-dimensional wind tunnel are investigated using the opposed flow velocity as a parameter. The gas and solid phase temperatures, preheat length and heat flux are considered in order to examine the flame ignition and spread characteristics. The numerical results reveal that the ignition delay time increases with the opposed flow velocity. However, the flame spread rate varies with the opposed flow velocity in a non-monotonic manner that can be identified as two distinct regimes with a peak value in between. The flame spread rate reaches a maximum at V g = 32cm/s, and then falls, regardless of whether the flow velocity is increasing or decreasing. For V g < 32cm/s, the flame behaviors are dominated by oxygen transport. For V g > 32cm/s, the flame stretch effect controls the flame behavior. Additionally, this work demonstrates that the effect of radiation delays the flame ignition and reduces both the flame strength and the corresponding spread rate. The effects of the opposed flow temperature and thickness of the solid fuel are also investigated. The predicted results indicate that a higher opposed flow temperature or a thinner solid fuel facilitates the ignition and accelerates flame spread. The effect of opposed flow velocity eventually overcomes that of opposed flow temperature as the flow speed is further increased.